Surgical instrument

ABSTRACT

An electrosurgical cutting blade ( 1 ) comprises a first electrode ( 2 ), a second electrode ( 3 ), and an electrical insulator ( 4 ) separating the first and second electrodes. The first and second electrodes ( 2  and  3 ) have dissimilar characteristics (cross-sectional area, thermal conductivity etc.) such that the first electrode ( 2 ) is encouraged to become an active electrode and the second electrode ( 3 ) is encouraged to become a return electrode. The spacing between the first and second electrodes (between 0.25 mm and 3.0 mm) and the peak voltage supplied to the electrodes are both selected such that arcing does not occur directly between the electrodes, but between the first electrode and the tissue at the target site. The arrangement is such that, in use, a thermal differential of at least 50° C. is established between the first and second electrodes, such that the second electrode is maintained below a temperature of 70° C. This is achieved either by thermally insulating the second electrode from the first electrode, and/or by transferring heat away from the second electrode, e.g. by conduction, forced cooling, or by means of a heat pipe.

[0001] This invention relates to a bipolar electrosurgical cutting device such as a scalpel blade, and to an electrosurgical system comprising an electrosurgical generator and a bipolar electrosurgical cutting device. Such systems are commonly used for the cutting of tissue in surgical intervention, most commonly in “keyhole” or minimally invasive surgery, but also in “open” surgery.

[0002] Electrosurgical cutting devices generally fall into two categories, monopolar and bipolar. In a monopolar device, a radio frequency signal is supplied to an active electrode which is used to cut tissue at the target site, an electrical circuit being completed by a grounding pad which is generally a large area pad attached to the patient at a location remote from the target site. In contrast, in a bipolar arrangement, both an active and a return electrode are present on the cutting device, and the current flows from the active electrode to the return electrode, often by way of an arc formed therebetween.

[0003] An early example of a bipolar RF cutting device is described in U.S. Pat. No. 4,706,667 issued to Roos, in which the return or “neutral” electrode is set back from the active electrode. Details for the areas of the cutting and neutral electrodes are given, and the neutral electrode is said to be perpendicularly spaced from the active electrode by between 5 and 15 mm. In a series of patents including U.S. Pat. No. 3,970,088, U.S. Pat. No. 3,987,795 and U.S. Pat. No. 4,043,342, Morrison describes a cutting/coagulation device which has “sesquipolar” electrode structures. These devices are said to be a cross between monopolar and bipolar devices, with return electrodes which are carried on the cutting instrument, but which are preferably between 3 and 50 times larger in area than the cutting electrode. In one example (U.S. Pat. No. 3,970,088), the active electrode is covered with a porous, electrically-insulating layer, separating the active electrode from the tissue to be treated and causing arcing between the electrode and the tissue. The insulating layer is said to be between 0.125 and 0.25 mm (0.005 and 0.01 inches) in thickness.

[0004] In another series of patents (including U.S. Pat. No. 4,674,498, U.S. Pat. No. 4,850,353, U.S. Pat. No. 4,862,890 and U.S. Pat. No. 4,958,539), Stasz proposed a variety of cutting blade designs. These were designed with relatively small gaps between two electrodes such that arcing would occur therebetween when an RF signal was applied to the blade, the arcing causing the cutting of the tissue. Because arcing was designed to occur between the electrodes, the typical thickness for the insulating material separating the electrodes was between 0.025 and 0.075 mm (0.001 and 0.003 inches).

[0005] The present invention seeks to provide a bipolar cutting blade which is an improvement over the prior art.

[0006] Accordingly, there is provided an electrosurgical system comprising a bipolar cutting blade, a handpiece to which the cutting blade is secured, and an electrosurgical generator for supplying a radio frequency voltage signal to the cutting blade, the cutting blade comprising first and second electrodes, and an electrical insulator spacing apart the electrodes, the spacing being between 0.25 mm and 3.0 mm, and the electrosurgical generator being adapted to supply a radio frequency voltage signal to the cutting blade which has a substantially constant peak voltage value, the relationship between the peak voltage value and the spacing between the electrodes being such that the electric field intensity between the electrodes is between 0.1 volts/μm and 2.0 volts/μm, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode.

[0007] The term “blade” is herein meant to include all devices which are designed such that both the active cutting electrode and the return electrode are designed to enter the incision made by the instrument. It is not necessary that the cutting device is only capable of making an axial incision, and indeed it will be shown below that embodiments of the present invention are capable of removing tissue in a lateral direction.

[0008] The first important feature of the present invention is that the spacing between the electrodes and the electric field intensity therebetween is carefully controlled such that there is no direct arcing between the electrodes in the absence of tissue. The electric field intensity between the electrodes is preferably between 0.15 volts/μm and 1.1 volts/μm, and typically between 0.2 volts/μm and 1.1 volts/μm. In one preferred arrangement, the spacing between the first and second electrodes is between 0.25 mm and 1.0 mm, and the electric field intensity between the electrodes is between 0.33 volts/μm and 1.1 volts/μm. This ensures that the field intensity is sufficient for arcing to occur between the first electrode and the tissue, but not directly between the first and second electrodes.

[0009] However, even where direct arcing between the electrodes is prevented, there is still a potential problem if the two electrodes are similar in design. In a bipolar cutting device, only one of the electrodes will assume a high potential to tissue (and become the “active” electrode), with the remaining electrode assuming virtually the same potential as the tissue (becoming the “return” electrode). Where the first and second electrodes are similar, which electrode becomes the active can be a matter of circumstance. Usually, whichever electrode first contacts the tissue will become the return electrode, with the other electrode becoming the active electrode. This means that, in some circumstances, one electrode will be the active electrode, and at other times the other electrode will be the active electrode. Not only does this make the device difficult for the surgeon to control (as it will be uncertain as to exactly where the cutting action will occur), but as it is likely that any particular electrode will at some time have been active.

[0010] When an electrode is active, there is a build up of condensation products on the surface thereof. This is not a problem when the electrode continues to be the active electrode, but it does make the electrode unsuitable for use as a return electrode. Thus, in the instance where two similar electrodes are employed, it is likely that, as each will at some times become active electrode and at other times the return electrode, the build up of products on both electrodes will lead to a decrease in performance of the instrument. Therefore, the present invention provides that the first electrode has a characteristic which is dissimilar from that of the second electrode, in order to encourage one electrode to assume preferentially the role of the active electrode.

[0011] The characteristic of the first electrode which is dissimilar from that of the second electrode conveniently comprises the cross-sectional area of the electrode, the cross-sectional area of the first electrode being substantially smaller than that of the second electrode. This will help to ensure that the first electrode (being of a smaller cross-sectional area) will experience a relatively high initial impedance on contact with tissue, while the relatively larger area second electrode will experience a relatively lower initial impedance on contact with tissue. This arrangement will assist in encouraging the first electrode to become the active electrode and the second electrode to become the return electrode.

[0012] The characteristic of the first electrode which is dissimilar from that of the second electrode alternatively or additionally comprises the thermal conductivity of the electrode, the thermal conductivity of the first electrode being substantially lower than that of the second electrode. In addition to the initial impedance, the rate of rise of the impedance is a factor influencing which electrode will become active. The impedance will rise with desiccation of the tissue, and the rate of desiccation will be influenced by the temperature of the electrode. By selecting an electrode material with a relatively low thermal conductivity, the electrode temperature will rise quickly as little heat is conducted away from the part of the electrode at which the arc is forming. This will ensure a relatively fast desiccation rate, producing a correspondingly fast rise in impedance and ensuring that the first electrode remains the active electrode.

[0013] The characteristic of the first electrode which is dissimilar from that of the second electrode may further comprise the thermal capacity of the electrode, the thermal capacity of the first electrode being substantially lower than that of the second electrode. As before, a low thermal capacity helps to maintain the temperature of the first electrode at a relatively high level, ensuring that it remains the active electrode.

[0014] According to a further aspect of the invention, there is provided an electrosurgical system comprising a bipolar cutting blade, a handpiece to which the cutting blade is secured, and an electrosurgical generator for supplying a radio frequency voltage signal to the cutting blade, the cutting blade comprising first and second electrodes, and an electrical insulator spacing apart the electrodes, the spacing being between 0.25 mm and 1.0 mm, and the electrosurgical generator being adapted to supply a radio frequency voltage signal to the cutting blade which has a substantially constant peak voltage value, the peak voltage value being between 250 volts and 600 volts, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode.

[0015] Given a particular electrode separation, it is highly desirable that the generator delivers the same peak voltages despite varying load conditions. Heavy loading of the blade may otherwise make it stall (a collapse of the arc due to too low a voltage being applied), while light loading may otherwise result in voltage overshoots and direct arcing between the electrodes.

[0016] The invention also resides in a bipolar cutting blade comprising first and second electrodes and an electrical insulator spacing apart the electrodes, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode, the spacing between the electrodes being between 0.25 mm and 1.0 mm, such that, when the electrodes are in contact with tissue and an electrosurgical cutting voltage is applied therebetween, arcing does not occur directly between the electrodes, there also being provided means for ensuring that the temperature of the second electrode does not rise above 70° C.

[0017] As well as ensuring that the second electrode does not become active, it is also important to ensure that the temperature of the second electrode does not rise above 70° C., the temperature at which tissue will start to stick to the electrode. The means for ensuring that the temperature of the second electrode does not rise above 70° C. conveniently comprises means for minimising the transfer of heat from the first electrode to the second electrode. One way of achieving this is to ensure that the first electrode is formed from a material having a relatively poor thermal conductivity, preferably less than 20 W/m.K. By making the first electrode a poor thermal conductor, heat is not transferred effectively away from the active site of the electrode and across to the second electrode, thereby helping to prevent the temperature of the second electrode from rising.

[0018] Alternatively or additionally, the heat can be inhibited from transferring from the first electrode to the second electrode by making the electrical insulator separating the electrodes from a material having a relatively poor thermal conductivity, preferably less than 40 W/m.K. Again, this helps to prevent heat generated at the first electrode from transferring to the second electrode.

[0019] Another way of inhibiting the transfer of heat is to attach the first electrode to the electrical insulator in a discontinuous manner. Preferably, the first electrode is attached to the electrical insulator at one or more point contact locations, and/or is perforated with a plurality of holes such as to reduce the percentage contact with the electrical insulator.

[0020] A preferred material for the first electrode is tantalum. When tantalum is used for the active electrode, it quickly becomes coated with a layer of oxide material. This tantalum oxide is a poor electrical conductor, helping to ensure that the first electrode maintains its high impedance with respect to the tissue, and remains the active electrode.

[0021] Another way of helping to ensure that the temperature of the second electrode does not rise above 70° C. is to maximise the transfer of heat away from the second electrode. Thus any heat reaching the second electrode from the first electrode is quickly transferred away before the temperature of the second electrode rises inordinately. One way of achieving this is to form the second electrode from a material having a relatively high thermal conductivity, preferably greater than 150 W/m.K.

[0022] The second electrode may conveniently be provided with additional cooling means to remove heat therefrom, such as a heat pipe attached to the second electrode, or a cooling fluid constrained to flow along a pathway in contact with the second electrode. Whichever method is employed, it is advisable for there to be a temperature differential, in use, between the first and second electrodes of at least 50° C., and preferably of between 100 and 200° C.

[0023] Preferably, there is additionally provided a third electrode adapted to coagulate tissue. This coagulation electrode is conveniently attached to the second electrode with a further electrical insulator therebetween. It is necessary to ensure that the temperature of the coagulation electrode does not rise to too high a level, and so, if the coagulation electrode is attached to the second electrode (which is designed in accordance with the present teaching to be a good thermal conductor), it is preferable to arrange that heat is easily transferred across the further electrical insulator. This can be achieved by making the further insulator from a material having a relatively high thermal conductivity, or more typically, if the further insulator is not a good thermal conductor, by ensuring that the further insulator is relatively thin, typically no more than around 50 μm. In this way the transfer of heat across the further electrical insulator is greater than 5 mW/mm².K.

[0024] In one arrangement, the second and third electrodes are formed as conductive electrodes on an insulating substrate. Thus, both the second and third electrodes act as return electrodes when the blade is used to cut tissue with the first electrode. When the blade is used to coagulate tissue, a coagulating RF signal is applied between the second and third electrodes.

[0025] According to a final aspect of the invention, there is provided a method of cutting tissue at a target site comprising providing a bipolar cutting blade comprising first and second electrodes and an electrical insulator spacing apart the electrodes, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode; bringing the blade into position with respect to the target site such that the second electrode is in contact with tissue at the target site and the first electrode is adjacent thereto; supplying an electrosurgical cutting voltage to the cutting blade, the electrosurgical voltage and the spacing between the first and second electrodes being such that arcing does not occur in air between the first and second electrodes, but that arcing does occur between the first electrode and the tissue at the target site, current flowing through the tissue to the second electrode; and preventing heat build up at the second electrode such that the temperature of the second electrode does not rise above 70° C.

[0026] Preferably, the method is such that both the first and second electrodes come into contact with tissue at the target site substantially simultaneously.

[0027] The invention will now be described by way of example only, with reference to the accompanying drawings, in which:

[0028]FIG. 1 is a schematic diagram of an electrosurgical system in accordance with the present invention,

[0029]FIG. 2 is a schematic cross-sectional view of an electrosurgical cutting blade in accordance with the present invention,

[0030]FIG. 3 is a schematic diagram showing the lateral cutting action of the blade of FIG. 2,

[0031]FIGS. 4a to 4 d are schematic cross-sectional views of alternative embodiments of electrosurgical cutting blades in accordance with the invention,

[0032]FIGS. 5a and 5 b are schematic diagrams of electrosurgical cutting blades in accordance with the present invention, incorporating cooling means, and

[0033]FIGS. 6a and 6 b are alternative electrosurgical cutting blades in accordance with the present invention, incorporating an additional coagulation electrode.

[0034] Referring to FIG. 1, a generator 10 has an output socket 10S providing a radio frequency (RF) output for an instrument 12 via a connection cord 14. Activation of the generator 10 may be performed from the instrument 12 via a connection in cord 14 or by means of a footswitch unit 16, as shown, connected to the rear of the generator by a footswitch connection cord 18. In the illustrated embodiment, the footswitch unit 16 has two footswitches 16A and 16B for selecting a coagulation mode and a cutting mode of the generator 10 respectively. The generator front panel has push buttons 20 and 22 for respectively setting coagulation and cutting power levels, which are indicated in a display 24. Push buttons 26 are provided as an alternative means for selection between coagulation and cutting modes.

[0035] Referring to FIG. 2, the instrument 12 comprises a blade shown generally at 1 and including a generally flat first electrode 2, a larger second electrode 3 and an electrical insulator 4 separating the first and second electrodes. The first electrode 2 is formed of stainless steel having a thermal conductivity of 18 W/m.K, although alternative materials such as Nichrome alloy may also be used. The second electrode 3 is formed from a highly thermally-conducting material such as copper having a thermal conductivity of 400 W/m.K (alternative materials including silver or aluminium). The surface of the second electrode 3 is plated with a biocompatible material such as stainless steel, or alternatively with a non-oxidising material such as gold, platinum or palladium. The electrical insulator 4 is formed from a ceramic material such as AL203 which has a thermal conductivity of 30 W/m.K. Alternatively, foamed ceramic materials are available which have a substantially lower thermal conductivity.

[0036] A conductive lead 5 is connected to the first electrode 2, while a conductive lead 6 is connected to the second electrode 3. The RF output from the generator 10 is connected to the blade 1 via the leads 5 and 6 so that a radio frequency signal having a substantially constant peak voltage (typically around 400V) appears between the first and second electrodes. Referring to FIG. 3, when the blade 1 is brought into contact with tissue 7 at a target site, the RF voltage will cause arcing between one of the electrodes and the tissue surface. Because the first electrode 2 is smaller in cross-sectional area, and has a lower thermal capacity and conductivity than that of the second electrode 3, the first electrode will assume the role of the active electrode and arcing will occur from this electrode to the tissue 7. Electrical current will flow through the tissue 7 to the second electrode 3, which will assume the role of the return electrode. Cutting of the tissue will occur at the active electrode, and the blade 1 may be moved through the tissue. The blade 1 may be used to make an incision in the tissue 7, or moved laterally in the direction of the arrow 8 in FIG. 3 to remove a layer of tissue.

[0037] During cutting, considerable heat will be generated at the active electrode, and the electrode temperature may rise to 100-250° C. However, due to the poor thermal conductivity of the insulator 4, less heat is transmitted to the second electrode 3. Even when heat does reach the second electrode 3, the high thermal conductivity of the copper material means that much of the heat is conducted away from the electrode surface and into the body 9 of the electrode. This helps to ensure that a temperature differential is maintained between the first electrode 2 and the second electrode 3, and that the temperature of the second electrode remains below 70° C. for as long as possible. This ensures that the second electrode 3 remains the return electrode whenever the instrument 12 is activated, and also that tissue does not begin to stick to the electrode 3.

[0038] In addition to providing an insulator 4 which has a relatively low thermal conductivity, it is advantageous to ensure that the first electrode 2 contacts the insulator 4 as little as possible. In FIG. 2, the electrode 2 is not secured to the insulator 4 and electrode 3 in a continuous fashion, but by one or more point contact pins shown generally at 11. FIG. 4a shows a further design of blade in which the first electrode 2 is shaped so as to contact the insulator 4 only intermittently along its length, with regions 13 in which the electrode bows outwardly from the insulator 4. This helps to minimise further the transfer of heat from the first electrode 2, through the insulator 4, to the second electrode 3. FIG. 4b shows a further arrangement in which the first electrode 2 is provided with many perforations 15 such that it is in the form of a mesh. Once again, this helps to minimise the transfer of heat from the first electrode 2 to the insulator 4. FIG. 4c shows another arrangement in which there is an additional corrugated electrode layer 17 located between the first electrode 2 and the insulator 4. As before, this assists in helping to prevent heat generated at the first electrode 2 from reaching the second electrode 3, so as to maintain the thermal differential therebetween.

[0039]FIG. 4d shows a variation on the blade of FIG. 2, in which the blade is formed as a hook 19. The first electrode 2, the second electrode 3 and the insulator 4 are all hook-shaped, and the operation of the device is substantially as described with reference to FIG. 2. The hook electrode is particularly suited for parting tissue, whether used as a cold resection instrument without RF energisation, or as an RF cutting instrument. Tissue may be held in the angle 20 of the hook 19, while being manipulated or cut.

[0040] Whichever design of electrode is employed, it is advantageous if heat which does cross from the first electrode 2 to the second electrode 3 can be transferred away from the tissue contact surface of the electrode 3. In the blade of FIG. 2, the second electrode 3 is constituted by a relatively large mass of copper which is capable of conducting heat away from the electrode tip. The function of the electrode 3 can be further enhanced by employing cooling means as illustrated in FIGS. 5a and 5 b. In FIG. 5a, the second electrode 3 is attached to a heat pipe shown generally at 27. The heat pipe 27 comprises a hollow closed tube 28 with a distal end 29 adjacent to the electrode 3, and a proximal end 30 within the handpiece of the instrument 12. The tube 28 has a cavity 31 therein, containing a low boiling temperature liquid 32 such as acetone or alcohol. In use, heat from the electrode 3 causes the liquid 32 at the distal end 29 of the tube to vaporise, and this vapour subsequently condenses at the proximal end 30 of the tube because it is relatively cool with respect to the distal end 29. In this way, heat is transferred from the distal end of the electrode 3 to the proximal end thereof, from where it can be further dissipated by the handpiece of the instrument 12.

[0041]FIG. 5b shows an alternative arrangement in which the heat pipe of FIG. 5a is replaced with a forced cooling system shown generally at 33. The cooling system comprises a tube 34, again with a distal end 29 and a proximal end 30. The tube 34 includes a coaxial inner tube 35 defining an inner lumen 36 and an outer lumen 37. The inner tube is perforated towards the distal end of the tube, so that the inner and outer lumens are in communication one with another. In use, a self contained pump 38 causes a cooling fluid 39 to be circulated up the inner lumen 36 to the distal end 29, returning via the outer lumen 37 to be recirculated continuously. The circulating fluid is heated by the electrode 3, and the heat is taken by the fluid to the proximal end 30 of the tube 34. In this way, the second electrode 3 is kept cool, despite the elevated temperature at the first electrode 2.

[0042]FIGS. 6a and 6 b show arrangements in which a third electrode 40 is provided, in order to allow the coagulation or desiccation of the tissue 7. In FIG. 6a, a blade 1 is shown in accordance with the construction of FIG. 4b, and like parts are designated with like reference numerals. The third electrode 40 is attached to the second electrode 3, on the opposite side to that of the first electrode 2, and mounted on a further electrical insulator 41. RF signals may be supplied to the third electrode 40 from the generator 10 via a lead 42. The insulator 41 is formed from a thin layer of polyimide, the provision of a thin layer being because it has a poor thermal conductivity. The thin layer ensures that heat can transfer across the polyimide layer and that the coagulation electrode 40 can benefit from the thermal conductivity properties of the second electrode 3. In this way, the coagulation electrode 40 can remain relatively cool despite any heat previously generated by the first electrode 2. In use, tissue is cut as previously described. When it is desired to coagulate instead of cutting, the third electrode 40 is placed in contact with the tissue 7, and a coagulating RF signal is applied between the second electrode 3 and the third electrode 40.

[0043]FIG. 6b shows an alternative embodiment in which the second electrode 3 and third electrode 40 are metallised tracks on a substrate 43 of aluminium nitride material. As before, this material is electrically insulating yet a good thermal conductor, to allow for the conduction of heat away from the second and third electrodes.

[0044] The invention relies on the careful selection of a number of design parameters, including the spacing between the first and second electrodes, the voltage supplied thereto, the size and materials selected for the electrodes, and for the electrical insulator or insulators. This careful selection should ensure that there is no direct arcing between the electrodes, that only one electrode is encouraged to be the active electrode, and that the return electrode is kept cool either by preventing heat reaching it and/or by transferring heat away from it should the heat reach the second electrode.

[0045] The relatively cool return electrode ensures that there is relatively little or no thermal damage to tissue adjacent the return of the instrument, while the tissue assists in the conduction of heat away from the return electrode. 

1. An electrosurgical system comprising a bipolar cutting blade, a handpiece to which the cutting blade is secured, and an electrosurgical generator for supplying a radio frequency voltage signal to the cutting blade, the cutting blade comprising first and second electrodes, and an electrical insulator spacing apart the electrodes, the spacing being between 0.25 mm and 3.0 mm, and the electrosurgical generator being adapted to supply a radio frequency voltage signal to the cutting blade which has a substantially constant peak voltage value, the relationship between the peak voltage value and the spacing between the electrodes being such that the electric field intensity between the electrodes is between 0.1 volts/μm and 2.0 volts/μm, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode.
 2. An electrosurgical system according to claim 1, wherein the characteristic of the first electrode which is dissimilar from that of the second electrode comprises the cross-sectional area of the electrode, the cross-sectional area of the first electrode being substantially smaller than that of the second electrode.
 3. An electrosurgical system according to claim 1, wherein the characteristic of the first electrode which is dissimilar from that of the second electrode comprises the thermal conductivity of the electrode, the thermal conductivity of the first electrode being substantially lower than that of the second electrode.
 4. An electrosurgical system according to claim 1, wherein the characteristic of the first electrode which is dissimilar from that of the second electrode comprises the thermal capacity of the electrode, the thermal capacity of the first electrode being substantially lower than that of the second electrode.
 5. An electrosurgical system according to claim 1, wherein the electric field intensity between the electrodes is between 0.15 volts/μm and 1.1 volts/μm.
 6. An electrosurgical system according to claim 5, wherein the electric field intensity between the electrodes is between 0.2 volts/μm and 1.1 volts/μm.
 7. An electrosurgical system according to claim 1, wherein the spacing between the first and second electrodes is between 0.25 mm and 1.0 mm, and the electric field intensity between the electrodes is between 0.33 volts/μm and 1.1 volts/μm.
 8. An electrosurgical system comprising a bipolar cutting blade, a handpiece to which the cutting blade is secured, and an electrosurgical generator for supplying a radio frequency voltage signal to the cutting blade, the cutting blade comprising first and second electrodes, and an electrical insulator spacing apart the electrodes, the spacing being between 0.25 mm and 1.0 mm, and the electrosurgical generator being adapted to supply a radio frequency voltage signal to the cutting blade which has a substantially constant peak voltage value, the peak voltage value being between 250 volts and 600 volts, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode.
 9. A bipolar cutting blade comprising first and second electrodes and an electrical insulator spacing apart the electrodes, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode, the spacing between the electrodes being between 0.25 mm and 1.0 mm, such that, when the electrodes are in contact with tissue and an electrosurgical cutting voltage is applied therebetween, arcing does not occur directly between the electrodes, there also being provided means for ensuring that the temperature of the second electrode does not rise above 70° C.
 10. A bipolar cutting blade according to claim 9, wherein the means for ensuring that the temperature of the second electrode does not rise above 70° C. comprises means for minimising the transfer of heat from the first electrode to the second electrode.
 11. A bipolar cutting blade according to claim 10, wherein the first electrode is formed from a material having a relatively poor thermal conductivity of less than 20 W/m.K.
 12. A bipolar cutting blade according to claim 10, wherein the electrical insulator is formed from a material having a relatively poor thermal conductivity of less than 40 W/m.K.
 13. A bipolar cutting blade according to claim 10, wherein the first electrode is attached to the electrical insulator in a discontinuous manner.
 14. A bipolar cutting blade according to claim 13, wherein the first electrode is attached to the electrical insulator at one or more point contact locations.
 15. A bipolar cutting blade according to claim 13, wherein the first electrode is perforated with a plurality of holes such as to reduce the percentage contact with the electrical insulator.
 16. A bipolar cutting blade according to claim 9, wherein the means for ensuring that the temperature of the second electrode does not rise above 70° C. comprises means for maximising the transfer of heat away from the second electrode.
 17. A bipolar cutting blade according to claim 16, wherein the second electrode is formed from a material having a relatively high thermal conductivity of greater than 150 W/m.K.
 18. A bipolar cutting blade according to claim 16, wherein the second electrode is provided with additional cooling means to remove heat therefrom.
 19. A bipolar cutting blade according to claim 18, wherein the cooling means comprises a heat pipe attached to the second electrode.
 20. A bipolar cutting blade according to claim 18, wherein the cooling means comprises a cooling fluid constrained to flow along a pathway in contact with the second electrode.
 21. A bipolar cutting blade according to claim 9, wherein the arrangement is such that there is a temperature differential, in use, between the first and second electrodes of at least 50° C.
 22. A bipolar cutting blade according to claim 9, wherein there is additionally provided a third electrode adapted to coagulate tissue.
 23. A bipolar cutting blade according to claim 22, wherein the third electrode is attached to the second electrode with a further electrical insulator therebetween.
 24. A bipolar cutting blade according to claim 23, wherein the arrangement is such that the transfer of heat across the further electrical insulator is greater than 5 mW/mm².K.
 25. A bipolar cutting blade according to claim 9, wherein at least the first electrode is formed of tantalum.
 26. A method of cutting tissue at a target site comprising providing a bipolar cutting blade comprising first and second electrodes and an electrical insulator spacing apart the electrodes, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode; bringing the blade into position with respect to the target site such that the second electrode is in contact with tissue at the target site and the first electrode is adjacent thereto; supplying an electrosurgical cutting voltage to the cutting blade, the electrosurgical voltage and the spacing between the first and second electrodes being such that arcing does not occur in air between the first and second electrodes, but that arcing does occur between the first electrode and the tissue at the target site, current flowing through the tissue to the second electrode; and preventing heat build up at the second electrode such that the temperature of the second electrode does not rise above 70° C. 